The surfaces of all eukaryotic cells are richly endowed with complex glycoconjugates. Cell surface glycans, together with the protein and lipid backbones to which they are linked, form the interfaces at which cell-cell interactions occur. Consistent with their subcellular location and immense structural diversity, specific glycans function as cell surface tags that allow cells to appropriately interact with each other and with their local environment. Therefore, mechanisms that control glycan expression also control developmental, immunological, and pathological cellular behaviors. Using genetic and molecular techniques, we discovered that a Drosophila Toll-like receptor (TLR), called Tollo/Toll-8, is a key component of a mechanism that leads to tissue-specific glycan expression. Tollo activity induces the neural-specific expression of a set of N-linked oligosaccharides known as the HRP-epitope. Tollo is not expressed on neurons but is found on ectodermal cells that contact differentiating neurons. This new function for a TLR provides the means to understand how cell-cell interactions influence tissue-specific glycosylation. The molecular pathway through which the Tollo signal is propagated will be identified and characterized by generating new mutations that affect expression of the HRP-epitope. In addition to our mutagenesis screen, we will determine whether previously identified genes, that are components of TLR signaling pathways in other contexts, are also active in regulating glycan expression. To place new genes in context and to determine the cellular mechanism of Tollo function, the ability of altered forms of Tollo to induce neural-specific glycosylation will be investigated. Finally, the specificity of Tollo induced glycan expression will be assessed by determining the diversity of HRP-epitopes in the embryo and by characterizing the full extent of glycan changes caused by loss of Tollo function. The importance of specific glycosylation for normal tissue function indicates that mechanisms controlling glycan expression should ultimately provide new targets for therapeutic intervention in a broad range of human pathologies, including CNS injury, inflammatory/immune disorders, and tumor cell metastasis. [unreadable] [unreadable] [unreadable]